add_filter( 'wpv_filter_query', 'filter_empty_dates', 99, 3 );
    
function filter_empty_dates( $query_args,$view_settings ,$view_id ) {
       
    if ( $view_id == 503724) {
        $query_args['meta_query'] = array(
          'relation' => 'AND',
          array(
            'key' => 'wpcf-exam-date',
            'value' => '',
            'compare' => '!='
            ),
          array(
            'key' => 'wpcf-exam-date',
            'value' => time(),
            'compare' => '>='
            )
          );
    }
    return $query_args;
}